Fiber-optic light sources using optical fibers doped with rare-earth ions such as erbium (Er) and ytterbium (Yb) are used in a variety of applications such as material processing, telecommunications, spectroscopy, medicine, etc. In a configuration commonly referred to as MOPA (Master Oscillator/Power Amplifier), a relatively low-power fiber laser with a desired spectral and temporal properties is used to seed one or more subsequent fiber-optic amplification stages, which boost the power to a desired level. Characteristics of such fiber sources strongly depend on the design of optical fibers used in each stage, which are often optimized for a particular application or range of applications.
An optical fiber has a core, typically 5-100 micron (μm) in diameter, which guides light, and a cladding, typically 125-600 μm in diameter, which surrounds the core; the core has a higher refractive index than the cladding. The cladding is typically composed of fused silica, and the core typically includes dopants to raise the index relative to the cladding. These dopants can also impart other functionality to the core; in particular, inclusion of one or more rare-earth dopants enables the core to exhibit gain when optically pumped, typically using diode lasers, enabling fabrication of fiber lasers and amplifiers. For low-power fiber sources, for example having output power less than approximately 0.5 W, the pump light can be launched into the core because suitable pump diodes are available. For higher-power fiber sources, pump diodes with sufficient brightness to efficiently launch into the core are not available. Instead, the fiber cladding is surrounded by a lower-index material, typically a polymer or fluorosilicate glass, so that it also guides light. In this “double-clad fiber” (DCF) structure, the pump light is launched into the cladding, which is for DCFs is typically referred to as the “inner cladding” or “pump core”, but it is absorbed only in the core, retaining the benefits of a confined gain region. The advent of DCF has enabled power scaling of fiber sources to kW levels because high pump power can be launched into the relatively large area of the inner cladding.
Most fibers have a circular cladding shape, which is easiest to manufacture. In a rare-earth-doped DCF with a circular cladding, however, the pump absorption is observed to be relatively low because of the presence of “helical modes,” which propagate in a corkscrew-shaped pattern without intersecting the core and are thus not absorbed. In fact, in the absence of a mechanism to eliminate or scramble helical modes, a fraction of the pump light will not be absorbed even for extremely long fiber lengths, resulting in low system efficiency. Most DCFs therefore employ one or more mechanisms for scrambling helical modes. The most common approach is to use a shaped inner cladding; the most common shapes currently are octagonal and hexagonal, although many other shapes have been reported, including square, rectangular, D-shaped, and clover-shaped. Another approach is to route the fiber in a non-circular path, such as a figure-eight, but this method limits packaging options and entails other compromises. In polarization-maintaining (PM) and “holey” fibers, internal structures within the inner cladding effectively scramble helical modes, and the fiber cladding can thus be circular (although it doesn't have to be); most fiber sources, however, do not employ PM or holey fiber.
The MOPA system may employ the same gain fiber in both the laser/master oscillator (MO) and the power amplifier (PA) sections of the system, or may employ different fibers for the PA and MO sections. All published designs, to the knowledge of the inventors, employ some means to scramble helical modes in both the oscillator and amplifier fiber(s). In particular, most prior art designs employ shaped fibers in both the oscillator and amplifier(s), whether the core designs are the same or different in the oscillator and amplifier stages.
DCFs with shaped inner claddings, however, have certain practical disadvantages in comparison with more conventional circular-cladding fibers. Firstly, the shaped-cladding fiber is more expensive because of the additional processing steps involved in fabricating the shaped preform used to draw the fiber. Next, manufacturing tolerances are typically larger for shaped-cladding fibers. Specifically, higher variability exists in the cladding diameter and shape and in the core concentricity. These increased tolerances result in larger variability, both within a fiber run and lot-to-lot, in the pump loss when splicing to other fibers with matched cladding area, and in the pump absorption coefficient, which determines the optimum fiber length and/or the output power. Furthermore, stripping, cleaving, and splicing of shaped fibers has lower yield and is less reproducible, increasing manufacturing times and scrap because of required rework. Ultimately, higher pump and/or signal splice loss may be required to obtain acceptable yields or cycle times, resulting in lower system efficiency and problems associated with thermal management of the optical power lost in the splices, which is typically converted to heat.
Thus, there is a need for an improved cladding-pumped MOPA structure which addresses at least some of the aforementioned shortcomings.